The automation of automotive systems has become popular in many types of automobiles and other vehicles. Automotive moving systems such as Power Lift-gate, Power Window and Power Sliding Door include one or more brush type DC motors that require a motor speed control. The target of a motor speed control for the automation of automotive systems is not for achieving a constant motor speed, but for achieving motor speed profile in which the target speed is varied depending on the mechanical system's lateral or angular position.
In order to achieve a speed control, power applied to the motor needs to be controlled. The most commonly utilized is Pulse Width Modulation (PWM) control in which the applied motor voltage is switched at a fixed frequency while the duty cycle is varied by a microprocessor unit programmed to achieve the target speed control. A control module performs PWM control and supplies PWM power to the motor.
One of the side effects of PWM control is that it produces noise coming from mechanical system that becomes nuisance depending on its frequency. To eliminate audible noise, 20 KHz or a higher frequency is commonly used for PWM frequency. In conventional PWM control, PWM pulse train signal is transmitted to the motor though the wires. High frequency harmonic components of PWM pulse train signal in the motor interface wires cause AM band (500 KHz to 1.6 MHz) radiated emission (RE) problems in the case, PWM waveform has a such sharp edge to have shorter rise and fall time than 10 usec.
One method of preventing resonance or other vibration issues is to provide a PWM waveform slope dull enough to prevent RE EMC problems from occurring. In most of automotive moving system applications, in which max operating current is 10 A or greater, setting PWM slope dull enough to prevent RE EMC issue is not achievable without using a good thermal characteristic heat sink and a metal housing. Making a longer rise or fall time increases power dissipation of the FET and maximum applicable rise or fall time to set is limited by FET's thermal capability and the control's operating ambient temperature requirements.